Optical devices based on a light-modulating cell are well known and widely used as beam splitters, polarizing beam splitters, light valves, flat panel displays, switches, shutters, retarders, lenses, filters, etc. The conventional light-modulating cell is typically formed by a pair of glass substrates and an optical medium therebetween. At present, most of the existing embodiments of a light modulating cell employ a liquid crystal (LC) material as optical medium. Other kinds of substantially liquid optical mediums are also used with light modulating cells, for example, oriented organic materials, emulsion solutions, gels, etc. The use of emulsion solution is disclosed, for example, in the article "A Variable Light-Blocker", Linda Geppert, Editor, IEEE Spectrum, December 1997, p. 65.
The LC cell represents a birefringent medium effecting a double refraction of light impinging thereon. This property of the LC cell enables it to be employed in various optical devices, for example, in polarizer beam splitters (PBSs). The construction and operation of PBS device are known per se, being disclosed, for example, in the article "Using the Interface Between Glass and a Nematic Liquid Crystal for Optical-Radiation Polarization Over a Broad Spectral Range", A. A. Karetnikov, Opt. Spectrosk, (USSR), 67, 324-326 , August 1989. As illustrated in FIG. 1, a conventional PBS device, generally designated 1, comprises LC cell 2 located between parallel sides 4a and 6a of a pair of glass prisms 4 and 6. The LC cell 2 typically comprises a layer 8 formed of a nematic liquid crystal (NLC) material, which is enclosed between two so-called "orienting layers" 10a and 10b formed on the outer surfaces of the sides 4a and 6a. Beam propagation through the PBS device 1 and the polarization separation produced thereby are shown in FIG. 1 in a self-explanatory manner and therefore need not be specifically described.
A common drawback of such LC cell based PBS devices is the unavoidable requirement of substantially stable and constant thermal surroundings. A PBS device typically provides high polarization purity, unless subjected to thermal variations.
A similar problem exists in other LC cell based applications. For example, flat-panel liquid crystal displays (LCDs), which, due to their low drive voltage, low power consumption and readability in bright ambient light, have become very popular and are widely used in watches, calculators, games, information boards, portable computers, pocket television, etc. A common arrangement of LCD is illustrated in FIGS. 2a and 2b, showing the main principles of LCD technology based on so called twisted NLC cell. LCD, generally designated 22, comprises a light source 24, and LC cell, generally at 26. The light source 24 provides a back illumination of the LC cell 26. The cell 26 includes LC layer 26a, disposed within a space between two parallel glass plates 28 and 30. The glass plates 28 and 30, functioning as transparent electrodes, are formed with polarizing films 32 (polarizing) and 34 (analyzing), respectively, on their outer surfaces. The operation of LCD is known per se, being shown in the drawings in a self-explanatory manner, and is therefore, not specifically described. One of the essential limitations of LCDs of the kind specified above consists of the susceptibility of the cell 26 to thermal variations.
LCDs based on scattering properties of microlens effects have been developed and disclosed, for example, in T. Nose et. al. Proc. Of the SID, Vol. 32/3, 1991. The main problem of such devices is their susceptibility to pressure variation due to molecular reorientation.
Replacing one of the LC cell glass substrates with a plastic substrate as suggested by N. Wakita at. el. at the FLC 93 PROGRAM conference in Tokyo, pp. 367-368, significantly improves weight and cost parameters of the LCD and increases its flexibility and mechanical shock resistivity. However, susceptibility to thermal variation almost does not change.
Bistable Ferroelectric LC (FLC) based devices are well known, being disclosed, for example, in "Ferroelectric Liquid Crystals in High Information Content Displays", C. Escher and R. Wingen, Adv. Matter 4 (1992) No. 3. Such devices are based on a special structure of FLC that enables fast switching. The method of oriented FLC cell preparation based on smectic A smectic C phase transition in responsive to density variations of the FLC material. The FLC material volume changes during cell preparation, thus causing pressure changes within the FLC cell, which, in turn, result in the creation of zig-zag like defects which subsequently damage the cell. This is more specifically disclosed, for example, in "Ferroelectric Liquid Crystal Display with High Contrast Ratio", N. Yamamoto et. al. Jpn. J. Appl. Phys. Vol. 28 No. 3 March 1989, 524, "Electro-Optic Switching Using TIR by a Ferroelectric Liquid Crystal", M. R. Meadows at. al., Appl. Phys. Lett. 54 (15), 1989, "The Relationship Between the Physical Properties of the Alignment Layer and the Quality of SSFLC Cells", B. O. Myrvold, Mol. Cryst. Liq. Cryst. vol. 22, 1991).
At present, FLC mixtures are characterized by a temperature operating range at -10.degree. C.--55.degree. C. It is anticipated that in the future, FLC mixtures with ferroelectric phase between -30.degree. C. and 70.degree. C. and a fast response time will be realized. It is known that FLC material is characterized by such properties as tilt angle and temperature dependence. It was found that a tilt angle close to 22.5.degree. C. with weak temperature dependence is the optimal condition for achieving the best contrast of FLCDs. Such trends emphasize FLC device susceptibility to thermal variations.
Laser addressed LCDs have been developed and disclosed, for example, in "Thermal Properties and Heat Flow in the Laser Addressed Liquid-Crystal Display", D. Armitage, J. Appl. Phys. 52 (3), March 1981. The operation of a laser addressed LCD is based on the local heating of its LC cell to manipulate pixels. Obviously, such thermal changes constrain the deterioration of the LC cell condition and damage the LC cell's performance.
A tunable wavelength-selective filter is an extremely important device for wavelength-division-multiplexing systems. Wide versions of electrically tunable filters have been actively studies. For example, the LC Fabry-Perot interferometer stands out for its compact size, low driving voltage, narrow band-pass, low loss, and wide tuning range filter as proposed in "Pigtailed compact tunable wavelength-selective filter using a LC for wavelength-division-multiplexing" Jpn. J. Appl. Phys. Vol. 31 (1992) pp. 1355-1357. This type of LC cell based device is also susceptible to thermal variations.
Generally, it is often the case that a light-modulating cell base device is part of the optical system and, therefore, should meet the requirements associated with operating under various thermal conditions. However, the light-modulating cell exposed to thermal variations experiences, inter alia, pressure changes within the cell. Such pressure changes involve density variations of the LC material and expansion or compression of the cell's walls. As a result, the LC based device and the entire optical system employing the same will exhibit performance deterioration.